The objectives of this project are the determination of the nature and biological properties of rickettsial antigens and constituents and the development of procedures for classification of spotted fever group rickettsiae. The serological relationship of the spotted fever group antigen and the erythrocyte sensitizing substance was determined by tandem crossed immunoelectrophoresis. Evidence of the structure and chemical nature of the group antigen was obtained by a combination of immunological and chemical methods. Antibodies to particular Rickettsia rickettsii antigens were demonstrated in the sera of spotted fever patients by crossed immunoelectrophoresis.